Birefringent optical correlator

ABSTRACT

A birefringent optical correlator having a light source, a pair of ultrasonic delay lines and a photosensitive device. The birefringent optical correlator does not contain apertures or polarizers between the delay lines and therefore passes the entire image therethrough. A polarizer is located before the first delay line and after the second delay line in order to separate the modulated or diffracted light. Thus, the birefringent optical correlator requires substantially little source light intensity and mechanical alignment.

United State:

Hileman et al.

BIREFRINGENT OPTICAL CORRELATOR [75] Inventors: Ronald E. Hileman, Boulder, Colo; James T. Campbell, Sherborn, Mass.

[73] Assignee: The United States of America as represented by the Secretary of the Air Force [22] Filed: March 11,1970

[21] Appl. No.: 24,936

[52] U.S.Cl. ..235/l81,343/100 CL,350/149, 350/151, 350/161, 324/77 K [51] Int. Cl ..G06f 15/34, 606g 7/19, GOlr 23/16 [58] Field of Search .343/100 CL; 235/181; 350/149, 350/151,161;324/77 K [56] References Cited UNITED STATES PATENTS 3,457,425 7/1969 Preston ..350/149 UX 3,596,182 7/1971 Menard ..350/16l X 3,111,666 11/1963 Wilmotte.... 235/181 X 3,441,724 4/1969 Taylor ..235/l81 3,280,318 10/1966 Gerig et a1 235/181 3,573,449 4/1971 Maloney .235/181 3,432,647 3/1969 Wilmotte ..235/181 3,483,386 12/1969 .lernigan ..350/l6l X 1 51 Jan. 30, 1973 3,055,258 9/1962 Hurvitz ..350/161 X 3,474,286 10/1969 Hergenrother ..235/l8l X 2,418,964 4/1947 Arenberg ..350/149 X 3,509,453 4/1970 Wilmotte ..235/18l X 4/1963 Rosenthal ..350/16l X OTHER PUBLICATIONS IBM Technical Disclosure Bulletin, Lean, Vol. 1l,No. 8, Jan. 1969 pg. 899-901. IBM Technical Disclosure Bulletin, Johnson, Vol. 7,

' No. 2,July 1964 pg. 136.

Primary Examiner-Eugene G. Botz Assistant Examiner-James F. Gottman Attorney-Harry A. Herbert, Jr. and Jacob N. Erlich [57] ABSTRACT polarizer is located before the first delay line and afterv the second delay line in order to separate the modulated or diffracted light. Thus, the birefringent optical correlator requires substantially little source light intensity and mechanical alignment.

5 Claims, 3 Drawing Figures 3714404 OR IN235/181 BIREFRINGENT OPTICAL CORRELATOR BACKGROUND OF THE INVENTION This invention relates generally to correlators, and more particularly to a variable code optical correlator which requires less source light intensity and less critical mechanical alignment than previously existing correlators.

Optical correlators have been employed to correlate sequences with flow frequency to very high frequency bit rates at process gains beyond those achievable with conventional electronic circuitry. Variable code optical correlators use two acoustic delay lines, one for a locally generated reference sequence and another for the received signal. A transducer mounted at one end of each of the delay lines, changes the electrical signal to an acoustic shear wave which propagates to the far end of the line where it is terminated. Collimated, polarized light shines through the delay perpendicular to signal propagation.

The modulated light, collected and focused on a photo sensitive surface, experiences significant intensity variations when the acoustic signals in both delay lines are identical. The signals propagate in opposite directions in the parallel delay lines, sliding past one another, so that they are compared at all relative positions. The desired sequence is usually a biphase modulated binary code with several cycles of carrier per bit.

The modulated light may be shown to be the diffracted portion of the incident light. The image created by focusing the diffracted light may be described by a Fourier Transform between the space domain and the angle domain. correspondingly, the image of the binary sequence is a series of spectral lines spaced at angular intervals related to the sequence length and having an amplitude envelope defined by sin x/x function with nulls related to the length of an individual code bit.

Previous optical correlators have used double diffraction in which light reaching the photo-detector is diffracted by both delay lines consecutively. To separate the double diffracted light from the undiffracted, non-information-bearing light components, an

aperture was used between the delay lines. Since the intensity of light diffracted by a delay line at nominal drive levels is often less than 1 percent of the incident light, the double-diffracted image at the photo-detector contains less than (0.01) X (0.01) or percent of the incident light.

The aperture proceeding the photo multiplier must be quite small in double diffraction system since the center spectral line in the sin x/x envelope must be separated from neighboring lines to detect correlation at the theoretical process gain level. The angular separation of these lines may be shown to be equal to one light wave length divided by the delay line length; an extremely small angle requiring diffraction limited lenses and dimensional tolerances on the order of 10' inches. Thus, double diffraction systems are extremely difficult to align and maintain aligned.

SUMMARY OF THE INVENTION The birefringent optical correlator of this invention overcomes all of the problems set forth in detail hereinabove. The correlator is a two-delay line correlator which differs significantly from double defraction systems of the past in that the detected light is single diffracted. The birefringent optical correlator of the instant invention does not contain apertures or polarizers between the delay lines as in prior art correlators. This phase-birefringent optical correlator passes the entire image instead of the center spectral line in that image. Each of the delay lines contribute energy to the light intensity spectrum.

At the instant the signals are identical and perfectly aligned, the contributions from each are no longer of random phase, but are coherently in phase, so that the energy doubles. The linear polarizers serve to separate the modulated or diffracted light on the basis of its polarization orientation rather than diffraction angle, thus obviating any critical angular tolerance on the condensing lens.

The first delay line, illuminated by the incident collimated light, diffracts approximately 1 percent of the light into the first order image. The second delay line, illuminated by the diffracted light and 99 percent undiffracted light, also diffracts 1 percent of the previously undiffracted light into the first order image. Thus, the image intensity is approximately 2 percent, or 23 db stronger than the image intensity in the double diffraction system. Since photo multiplier signal to noise is proportional to light intensity, the birefringent system of this invention may use a much dimmer light source. The only angular tolerances are those requiring the light to pass perpendicularly across the sonic wave fronts in the delay lines which are often on the order of 1-5 minutes.

It is therefore an object of this invention to provide a birefringent optical correlator that requires less source light intensity and less critical mechanical alignment than were required in correlators of the past.

It is another object of this invention to provide a birefringent optical correlator in which temperature compensation, mechanical rigidity, size and weight can be reduced because the single diffraction optical correlator of this invention does not require diffraction.

It is another object of this invention to provide a birefringent optical correlator that is economical to produce and which utilizes conventional, currently available components that lend themselves to standard mass producing manufacturing techniques.

For a better understanding of the present invention, together with other and further objects thereof reference is made to the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

DESCRIPTION OF THE DRAWING FIG. I is a schematic diagram in perspective of the birefringent optical correlator of this invention;

FIG. 2 is a perspective view of a fused silica delay line utilized in the birefringent optical correlator of this invention; and

FIG. 3 is a schematic diagram of the orientation of the optic axes in relationship to the fused silica delay line of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1 of the drawing, the optical correlator 10 of this invention uses light modulation to detect the presence of the desired code sequence. Light modulation corresponding to the received electrical signal is achieved by ultrasonic delay lines 11 and 12. For clarity, only delay line 11 is shown in FIG. 2. It should be realized, however, that both delay lines 11 and 12 are identical. Referring now to FIG. 2, delay line 11 is a long, narrow acoustical medium with a transducer 14 at one end thereof and an absorber 16 at the other end. The electrical signal enters through the transducer 14 and is converted to an acoustical signal by the transducer 14. The acoustic signal propagates to the opposite end of the delay line 11 where it is prevented from reflecting by the absorber 16. The two most common delay line media for signals in the low frequency to very high frequency region are a water-alcohol solution delay line (not shown) and the fused silica glass delay lines 11 and 12 (shown in FIGS. 1 and 2). The water-alcohol solution of the delay line is contained in a long, narrow tank with optically clear and flat sides. The ethyl alcohol, 17 by volume, serves to compensate the temperature coefficient of the water. The fused silica delay line 11, shown in FIG. 2, is a long, narrow piece of optical clear glass. The total time delay of the optical correlator delay lines 11 and 12 are normally less than 100 sec. due to the combined effects of acoustic attenuation and aperture dimensions. The time delay of 100 p. sec. corresponds to fused silica and water-alcohol delay line lengths of approximately inches and 6 inches, respectively.

A liquid can support only compressional waves; whereas, a solid can support either compressional or shear waves. Therefore, the water-alcohol solution is always driven by compressional waves. The fused silica delay line 11 is generally driven by a shear wave. At any point in the water-alcohol delay line, the compressional or longitudinal wave causes the water solution to be compressed by an amount proportional to the instantaneous wave amplitude at that point. The compressional wave, because it uniformly compresses an elemental cube of the medium, does not generate any optic axes in the medium. When the delay line is driven by a sinusoidal signal, the compressional wave is cyclic so that areas of compression and rarefaction are interspersed along the length of the delay line.

The acoustic shear wave is a transverse wave. The shearing force is caused by the differential amplitude between adjacent portions of the acoustic transverse wave. The differential force causes opposite edges of an elemental cube in the medium to move in opposite directions. The shearing motion causes compression along one of the diagonals of the elemental cube. At the same instant, the opposite diagonal experiences rarefaction. The diagonals of the elemental cube correspond to the optic axes of a fused silica medium driven by a shear wave.

FIG. 3 illustrates the orientation of these optic axes in relationship to the fused silica delay line 11 as shown in FIG. 2. When the transducer 14 is driven by a sinusoidal signal, the shear wave is periodic so that in the orientation of either of the optic axes, areas of compression are interspersed with areas of rarefaction along the length of the delay line.

Referring again to FIG. 2, the acoustic signals in the delay line 11 are read out by illuminating the delay line 11 with collimated light 18 originating at a suitable source 20 such as a watt mercury arc lamp and shining through collimating lens 22, or as shown in FIG. 1, reflected of parabolic reflector 40 and shining perpendicular to the direction of acoustic wave propagation. The cyclic compression and rarefaction caused by a sinusoidal acoustic signal produces a corresponding change in medium density. Since light travels slower in a more dense medium, the changing medium density produces proportional delays on portions of the emergent wave of light. That portion of the light beam which enters an area of the delay line 11 under compression is delayed with respect to the neighboring portion of the light beam which traverses a region of delay line 11 experiencing rarefaction. In such a manner, the incident collimated light 18, for which the phase front is a plane, emerges with a corrugated phase front. The corrugated phase gives rise to the phenomena of birefringence and diffraction. A single corrugated phase front is present following the water-alcohol delay line; whereas two corrugated phase fronts (one along each of the optic axes) emerge from the fused silica delay line 11 utilized in this invention and shown in FIG. 2. The corrugated phase front light wave is actually a composite of three plane waves, each propagating in a slightly different direction at a slightly different frequency.

The light emerges from the first delay line 11 as three plane waves. Each of these waves is further modulated by a second delay line 12 (shown in FIG. 1) which is identical to delay line 11 and is made up of transducer 25 and absorber 26. The three components of the originally collimated light are each diffracted into three components by the signal in the second delay line 12. Note that the acoustic signal in the second delay line 12 propagates in a direction opposite to the signal in the first delay line 11. The motion causes one code, in effect, to scan the other. The effect on the opposite motions of the codes is to cause a light wave, frequency up-shifted by the first delay line 11 to be frequency down-shifted by delay line 12. 7

Referring once again to FIG. 1, the birefringent optical correlator 10 of this invention is made up of a plurality of mirrors 28, 30, 32, 34 and 36 which are used to fold the optical path of the light emanating from source 20. The arrangement of mirrors permits the optical correlator 10 to be mounted in any standard relay rack (not shown) with a base plate 38 being horizontal. An off-axis parabolic reflector 40 is utilized to collimate the light from source 20 just as collimating lens 22 in FIG. 2. Refraction optics are used throughout so that the broad spectrum of the light source does not cause chromatic aberration.

-A pair of ultrasonic delay lines 11 and 12 of any suitable configuration, for example, 18 XI X 2 inch blocks of fused silica may be utilized. These delay lines 11 and I2 are positioned perpendicular to the optic axis of the parabolic reflector 40. One of the delay lines 11 has a polarizer 24 located in front thereof. Polarizer 24 has its polarization axis either parallel or perpendicular to the direction of the sonic wave, that is, the polarization axis is either along the X or Y axis shown in FIG. 3. The delay line 11 is further mounted in such a position so that the transducer 14 of delay line 11 and transducer 25 of delay line 12 are exactly at a predetermined number of bits apart along a parallel axis. This exact spacing permits knowledge of the bit entering the delay line 11 containing the locally generated reference at the instant the single correlates.

The phase modulated light exiting delay line 12 is changed to intensity modulated light by the output linear polarizer 42 located after delay line 12 whose axis is cross (rotated 90) with respect to polarizer 24. A photo multiplier 44 receives the reflected light and produces an electrical replica of the light intensity modulation.

MODE OF OPERATION Referring to FIG. 1, the operation of the birefringent optical correlator of this invention is as follows. An electrical signal 46 enters delay line 11 and is converted to an acoustical signal by the transducer 14. The acoustic signal propagates to the opposite end of delay line 11 and is prevented from reflecting by the absorber 16. The acoustic signals in delay line 11 are read out by illuminating delay line 11 with a light 18 originating at source 20. The light 18 reflects off mirror 28 onto parabolic reflector 40 where it is collimated and then shines through polarizer 24 and delay line 11 in a direction perpendicular to the direction of the acoustic wave propagation. The cyclic compression and rarefaction caused by a sinusoidal acoustic signal produces a corresponding change in the medium density of delay line 11. Since light travels slower in a more dense medium, the change medium density produces proportional delays on portions of the emergent wave of light 18 exiting delay line 11. The light 18 emerging from first delay line 11 is further modulated by a second delay line 12. The phase modulated light 18 emerging from delay line 12 is changed to intensity modulated light by the output linear polarizer 42. The light 18 then reflects off mirror 30 to mirror 32 back to the parabolic reflector 40 onto mirror 34 and 36 until it reaches photo multiplier 44 which produces an electrical replica of the light intensity modulation.

Although the invention has been described with reference to a particular embodiment, it will be understood to those skilled in the art that the invention is capable of a variety of alternative embodiments within the spirit and scope of the appended claims.

We claim:

1. A birefringent optical correlator comprising a light source, a collimating means located adjacent said light source for collimating the light beam emanating from said source, a first ultrasonic delay line positioned adjacent said collimating means and perpendicular to the optic axis of said collimating means, a polarizer located between said collimating means and said first delay line, a second delay line directly adjacent said first delay line and parallel thereto and a photo sensitive device adjacent said second delay line'whereby said collimated light beam passes through said pair of delay lines and is received by said photo sensitive device.

2. A birefringent optical correlator as defined in claim 1 wherein said first and second delay lines are identical fused silica delay lines.

3. A birefringent optical correlator as defined in claim 2 wherein another polarizer is located directly adjacent said second delay line between said second delay line and said photo sensitive device.

4. A birefringent optical correlator as defined in claim 3 wherein each of said pair of delay lines has a transducer at one end thereo and an absorber at the other end, said first and second delay lines being 

1. A birefringent optical correlator comprising a light source, a collimating means located adjacent said light source for collimating the light beam emanating from said source, a first ultrasonic delay line positioned adjacent said collimating means and perpendicular to the optic axis of said collimating means, a polarizer located between said collimating means and said first delay line, a second delay line directly adjacent said first delay line and parallel thereto and a photo sensitive device adjacent said second delay line whereby said collimated light beam passes through said pair of delay lines and is received by said photo sensitive device.
 1. A birefringent optical correlator comprising a light source, a collimating means located adjacent said light source for collimating the light beam emanating from said source, a first ultrasonic delay line positioned adjacent said collimating means and perpendicular to the optic axis of said collimating means, a polarizer located between said collimating means and said first delay line, a second delay line directly adjacent said first delay line and parallel thereto and a photo sensitive device adjacent said second delay line whereby said collimated light beam passes through said pair of delay lines and is received by said photo sensitive device.
 2. A birefringent optical correlator as defined in claim 1 wherein said first and second delay lines are identical fused silica delay lines.
 3. A birefringent optical correlator as defined in claim 2 wherein another polarizer is located directly adjacent said second delay line between said second delay line and said photo sensitive device.
 4. A birefringent optical correlator as defined in claim 3 wherein each of said pair of delay lines has a transducer at one end thereof and an absorber at the other end, said first and second delay lines being mounted in such a position as to have their respective transducers located a predetermined number of bits apart along parallel axes. 